1. Field of the Invention
The present invention relates to for performing printing processing on multi-level image data in a high gradation manner with high precision.
2. Description of the Related Art
In an image input/output system, multi-level image data that is read by an input device, such as a scanner or a digital camera, is output to an output device, such as a printer or a display. The multi-level image data (for example, 256 levels for 8-bit precision) that is read by the input device is converted to image data with gray levels that can be output by the output device. The output data is represented as pseudo continuous tones using a process that is known as pseudo halftone processing.
In pseudo halftone processing, when the output device outputs only a binary image that is represented by dots that are either on or off, binarization processing is then performed. The binarization processing can be implemented with a dither method, an error diffusion method, and a minimum average error method. The dither method is faster. The error diffusion method and the minimum average error method produce good resolution and gradation.
The error diffusion method and the minimum average error method are theoretically very similar except that the timing of performing error diffusion is different between them. In addition to processing the binary image, the error diffusion processing includes multi-level error diffusion processing that is used for processing ternary or more gray levels. In a similar manner as in the binary error diffusion processing, the multi-level error diffusion processing can produce good resolution and gradation.
The output device outputs images with ternary or more gray levels using the various methods described below.
In inkjet printers, processes such as changing the diameter of dots, such as large, medium, and small dots, by controlling the amount of ink discharged from a print head, reproducing images with ternary or more gray levels by superimposing dots or using dark and light inks with different densities are undertaken. This makes it possible to produce images with ternary or more gray levels. A light ink obtained by diluting a dark ink in a range from ½ to ⅙ times the concentration of the dark ink typically used for this purpose.
In intaglio printing, such as gravure printing, an ink is applied to a printing plate in which depressions corresponding to matter to be printed are cut. In this case, the amount of ink should be transferred onto a sheet is controlled by changing the depth of those depressions. By controlling the amount of ink in this manner, it is possible to print images with ternary or more gray levels.
When printing is performed using printing presses, inkjet printers, and electrophotography, a dot gain phenomenon sometimes occurs. The dot gain phenomenon is a phenomenon in which dots become bigger than their desired size due to ink bleeding and/or ink spreading.
In error diffusion processing, errors that locally occur are diffused to neighboring pixels by performing a feedback control to maintain the density to a desired density. However, in high-density regions, dots become larger than their desired size due to dot gain phenomenon, whereby density saturation tends to occur in the high-density regions.
FIG. 9 is a graph that depicts the difference between input and output values. The dotted line indicates desired values with respect to an input value of 256 gray levels and the solid line indicates actual values. Even when brightness is required to be linearly output with 256 gray levels, as is indicated by the dotted line, it is clear from the graph that the result of the output value is actually like that indicated by the solid line due to dot gain. In other words, it is impossible to get the brightness that is the same as the desired brightness.
When performing error diffusion processing in typical printers, the processing is performed on an image that is subjected to gamma conversion in order to eliminate any effect caused by dot gain.
Even when errors are diffused, the effect of dot gain can be reduced by using real values obtained after the gamma conversion as shown in FIG. 10. Specifically, when a gray level of 1 is input to a printer, by performing error diffusion processing with a value of 0.52, it is possible to represent brightness or density corresponding to an input value of 1 that takes into consideration the effect of dot gain.
As shown in FIG. 10, values subjected to gamma conversion have decimal parts that are not integers. This requires a large memory capacity to save therein errors when computing values subjected to gamma conversion in the error-diffusion processing unit.
Accordingly, error diffusion processing is required in which both less memory space is used and the density saturation is reduced without deteriorating the gray levels of the original image.
An example of a known error diffusion technology that takes into consideration dot gain is disclosed in Japanese Patent Application Laid-open No. 2007-124195. In the technology disclosed in Japanese Patent Application Laid-open No. 2007-124195, when increasing the number of bits for each input value, an error-diffusion processing unit performs error diffusion by converting an input value of 8 bits into a value of 14 bits instead of simply multiplying the input value by 64, whereby the value is converted to a value that represents target brightness or density that takes into consideration dot gain. This makes it possible to obtain desirable gray levels.
In the technology disclosed in Japanese Patent Application Laid-open No. 2004-72293, the sum of coefficients to which diffuse errors are set is less than 1, whereby values of density are not stored by aiming at less density saturation in a high-density region.
In the dither method, because it is possible to design a system taking into consideration the effect of dot gain, there is no need to consider the above mentioned problems.
In the technology disclosed in Japanese Patent Application Laid-open No. 2007-124195, the memory capacity required for storing values that are obtained by performing error diffusion processing is larger than that when 8-bit values are input in error diffusion processing. This is so even though the required memory capacity is smaller than that when real numbers that are subjected to the gamma conversion in error diffusion processing are used as values to be saved.
In the technology disclosed in Japanese Patent Application Laid-open No. 2004-72293, density saturation rarely occurs in the high-density region; however, dot production is delayed in any highlighted region because errors are uniformly distributed with a value of less than 100%.
In the dither method, a rectangular super cell is generally used as a dither matrix. The super cell is formed such that a plurality of minimum unit cells used for dither processing is assembled. If the dither matrix with the super cell of size 32×32 is used, dot patterns of 1025 can be output. However, 1025 dot patterns are not required because a multi-level image, which is read using a commonly used input device, is formed of 8 bits (i.e., a value of 256).
The effect of dot gain can be reduced by selecting 256 dot patterns from among the gray levels that are capable of being output and that are obtained from the super cell of size 1025. The selection is done in such a manner that target brightness or density is obtained taking into consideration dot gain.